Properties of Supersonic Evershed Downflows

Abstract

We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe I 617.3 nm line with the CRISP instrument at the Swedish 1-m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes V zero-crossing wavelengths, and Stokes V maps in the far red wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the LOS velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime, even some of them are recurrents. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward moving grains in the mid and outer penumbra.

Magnetic Properties and Flow Angle of the Inverse Evershed Flow at Its Downflow Points

“…While umbral magnetic fields are barely inclined relative to the local surface normal and primarily expand into the upper solar atmosphere, the magnetic field lines in the penumbra show a mixture of different inclination angles (e.g., Solanki 2003;Bellot Rubio et al 2004;Beck 2008). Some of the magnetic field lines return back to the photosphere within the penumbra (del Toro Iniesta et al 2001;Franz & Schlichenmaier 2013; Ruiz Cobo & Asensio Ramos 2013; Esteban Pozuelo et al 2016), while some others continue upwards and outwards into the so-called superpenumbra that is seen in chromospheric diagnostics to extend beyond the sunspot boundary visible in the photosphere (Joshi et al 2017).…”

Three-lobed near-infrared Stokes V profiles in the quiet Sun

“…Moreover, many comparative and statistical studies between both spectral bands have been carried out at this angular resolution (Schlichenmaier & Collados 2002;Müller et al 2002;Borrero et al 2007;Borrero & Solanki 2010). At highspatial resolution (i.e., < 0.5") similar studies have been conducted in visible (Ichimoto et al 2007(Ichimoto et al , 2008Franz & Schlichenmaier 2013;Pozuelo et al 2016), and NIR spectral lines (Franz et al 2016).…”

Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST)

“…de la Cruz Rodríguez, 2015, 2016, and the references therein). Supersonic vertical velocities at the outer end of penumbral filaments have been reported (Figure 4) and can be associated with penumbral field lines dipping down below the solar surface (Tiwari et al, 2013;Esteban Pozuelo, Bellot Rubio, and de la Cruz Rodríguez, 2016), but similar flows have also been detected in the inner penumbra without relation to the Evershed flow (Louis et al, 2011). In both regions, very large field strengths (on the order of 7 kG and 5 kG, respectively, compared to typical peak umbral values of 2 -3 kG) have been associated with the high-speed flows (Siu-Tapia et al, 2017;Okamoto and Sakurai, 2018), but because the observed Stokes profiles show anomalous shapes, these determinations are uncertain.…”